The present invention relates to an ultrafine particulate zinc oxide which has a low coagulation of primary particles and coagulation can be reduced to such an extent that the particles can be very easily dispersed or suspended in an aqueous solvent even without passing through the process of grinding or after the slight dry grinding, and further which can be suitably used, for example, as a cosmetic powder and is favored with both transparency and ultraviolet-shielding ability. The present invention also relates to a production process of the ultrafine particulate zinc oxide.
Many kinds of zinc oxide powders have been heretofore put on the market and applied to an extender blended in cosmetic materials and the like. Known production processes for these zinc oxide powders are roughly classified into a liquid phase process and a gas phase process. In the liquid phase process, zinc oxalate, zinc hydroxide or basic zinc carbonate is synthesized, precipitated, separated by filtration with rinse and then thermally decomposed to obtain zinc oxide. The powder obtained has a specific surface area of 50 m2/g or more. The liquid phase process is, however, disadvantageous in that the productivity is low because a batch system is fundamentally used; the fine particles obtained are in a solid-liquid mixed phase state and must be subjected to filtration and drying for finishing as a product. Accordingly, the production cost can be hardly reduced; and due to impurities remaining after the synthesis, higher purity cannot be attained. The gas phase process includes a French process of oxidizing zinc vapor and an American process of oxidizing zinc vapor generated at the smelting process of zinc ore. In this gas phase process, a zinc oxide powder having a specific surface area of 30 m2/g or more can be obtained. Other than these, various proposals for improved processes have been made in recent years with an attempt to further increase the specific surface area and improve the purity.
However, the zinc oxide powder produced by the above-described conventional processes has a problem in that the primary particles are not uniform in shape and particle size and are readily coagulated to form huge secondary particles. In the case of using such a zinc oxide powder in cosmetic materials or the like, it is necessary to unbind the coagulated particles by cracking or grinding. At this time, mixing of impurities due to abrasion during the cracking or grinding process inevitably takes place, as a result, not only the purity decreases but also the powder cannot be prevented from lack of uniformity in the shape of particles, the sharpness of the particle size distribution, and the mean particle size. Accordingly, when such a powder is used as a cosmetic material, the cosmetic material suffers from poor touch feeling. Furthermore, due to a large amount of grinding energy required, the cost increases.
An object of the present invention is to provide an ultrafine particulate zinc oxide which has a low coagulation of primary particles and in the case of application to a cosmetic material or the like, can be stably dispersed or suspended as extremely fine particles in an aqueous solvent without passing through any process for unbinding the coagulation, such as cracking or grinding, or after the slight dry grinding.
Another object of the present invention is to provide a production process of ultrafine particulate zinc oxide.
Still another object of the present invention is to provide a cosmetic material having both transparency and ultraviolet-shielding ability.
The present invention has been made under these circumstances and relates to an ultrafine particulate zinc oxide having a specific surface area determined by the single-point BET technique of from about 10 to about 200 m2/g and having a substantially isotropic primary particle shape, wherein the specific volume determined by a tapping machine is from about 4 to about 40 ml/g.